This invention relates to apparatus for scraping the inner surface of a wellbore.
This invention relates to apparatus for scraping the inner surface of a wellbore.
It is well known in the gas and oil drilling industry to run a scraper assembly down a wellbore so as to clean the inner surface of the wellbore casing wall. This operation is typically undertaken when there is a need to grip the inner surface of the wellbore casing with apparatus such as an inflatable packer. Naturally, the effectiveness of the apparatus gripping the casing is improved if the portion of casing to be gripped is substantially clean and free of loose fragments. In a conventional operation, a scraper assembly is attached to the bottom of the gripping apparatus so that cleaning of the casing may be completed as the gripping apparatus is run to the required depth. The scraping and gripping functions may be thereby executed in a single run.
A conventional scraper assembly is shown in FIG. 1 of the accompanying drawings. Typically, a prior art assembly incorporates a plurality of scraper elements mounted with compression springs about a mandrel. The scraper elements are arranged in such a way as to ensure full circumferential scraping of the casing when the assembly is run downhole without rotation. In the assembly of FIG. 1, this is achieved with the use of three longitudinally spaced pairs of scraper elements which are circumferentially offset relative to each other. A small degree of circumferential overlap is provided between the pairs of scraper elements so as to ensure uninterrupted circumferential scraping. Each scraper element covers approximately 60xc2x0 of the circumference of wellbore casing to be scraped. The scraper elements of each pair are located on opposite sides of the mandrel and are biased radially into scraping engagement with the wellbore casing by means of compression springs.
A number of problems are associated with the conventional scraper assembly described above. Firstly, the assembly is undesirably long due to the longitudinal spacing of the scraper element pairs. This longitudinal spacing is necessitated by the spring biasing system employed and the need to circumferentially overlap the pairs of scraper elements so as to ensure full scraping of the wellbore. Secondly, the multiple scraper element arrangement results in an item of downhole equipment which is relatively complex and expensive to manufacture.
It is an object of the present invention to provide a downhole scraper assembly which has a relatively short length whilst providing a full circumferential scraping capability.
It is a further object of the present invention to provide a scraper assembly which is relatively convenient and inexpensive to manufacture.
It is yet a further object of the present invention to provide a scraper assembly which is reliable and which is sufficiently inexpensive to manufacture for it to be considered as readily disposable.
The present invention provides a scraper assembly for use in a wellbore, the scraper assembly comprising a scraper element incorporating: a generally cylindrical member defined by a wall having a slot extending through the wall thickness; and at least one tooth member provided on the outer surface of the wall for scraping engagement with a wellbore, the scraper assembly being characterised in that the slot extends helically along the length of the cylindrical member.
The scraper assembly of the present invention may thereby incorporate only one scraper element to ensure full circumferential scraping. The slot in the wall of the generally cylindrical member allows for radial deflection of the scraper element as the at least one tooth member engages the wellbore. The scraper element is sized so that the maximum diameter of the scraper element (as determined by the at least one tooth member), when in its relaxed state prior to use, is greater than the inner diameter of the wellbore casing to be scraped. Thus, as the scraper assembly of the present invention is pressed downhole, the at least one tooth member is deflected radially inward. The slot allows the radial deflection without undesirable buckling of the scraper element. Furthermore. the arrangement is such that the deflection is elastic. This results in the at least one tooth member applying an appropriate radial force on the wellbore casing during the scraping process.
Preferably, four tooth members are provided on the outer surface of the wall for scraping engagement with a wellbore. It is desirable for the or each tooth member to extend helically about the longitudinal axis of the scraper element. Furthermore, it is preferable for the slot to extend from one end of the generally cylindrical member to the opposite end of the generally cylindrical member. The slot may also extend helically along the length of the generally cylindrical member. It is also desirable for the or each tooth member to be defined on a central portion of the generally cylindrical member so as to provide end portions of the generally cylindrical member for mounting the scraper element adjacent a body member. The mounting of the scraper element adjacent the body member preferably permits radial deformation of the full length of the scraper element.
Furthermore, it is preferable for the scraper element to be configured so that, when radially deformed by a wellbore casing in use, the or each tooth member has a circular or part circular profile when viewed along the longitudinal axis of the scraper element and the outer diameter of this profile is equal to the inner diameter of the wellbore casing.
It is also desirable to provide the scraper element with at least one further slot which extends through the wall thickness, a portion of the at least one further slot extending helically along the scraper element and a portion of the at least one further slot extending in a circumferential direction at each end of the helically extending portion. It may also be preferable to provide at least one groove on the outer surface of the wall, the at least one groove extending helically along the length of the scraper element from one end of the scraper element to the opposite end of the scraper element. This at least one groove provides a fluid way which allows the passage of wellbore fluid past the scraper assembly when in use.
Thus, the scraper assembly of the present invention has the advantage of being relatively short in comparison to conventional scraper assemblies whilst providing a full circumferential scraping capability. Furthermore, since the inherent resilience of the scraper element is harnessed so as to obviate the need for discrete compression springs and since full circumferential scraping is provided by a single scraper element, the scraper assembly of the present invention is relatively convenient and inexpensive to manufacture and may be considered as a disposable item of downhole equipment.
Embodiments of the invention will now be described with reference to the accompanying drawings.